Method for creating a micromechanical membrane structure and mems component

ABSTRACT

In a method for manufacturing a micromechanical membrane structure, a doped area is created in the front side of a silicon substrate, the depth of which doped area corresponds to the intended membrane thickness, and the lateral extent of which doped area covers at least the intended membrane surface area. In addition, in a DRIE (deep reactive ion etching) process applied to the back side of the silicon substrate, a cavity is created beneath the doped area, which DRIE process is aborted before the cavity reaches the doped area. The cavity is then deepened in a KOH etching process in which the doped substrate area functions as an etch stop, so that the doped substrate area remains as a basic membrane over the cavity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for creating a micromechanicalmembrane structure in a silicon substrate in which a cavity is createdin the substrate's back side in a DRIE (deep reactive ion etching)process, and the present invention further relates to MEMS componentsmanufactured using the method according to the present invention.

2. Description of the Related Art

The DRIB process is described in published German Patent document DE 4241 045 C1 as a method for anisotropic etching of silicon substrates. Thegoal of this method is to perform the etching only at a right angle tothe substrate surface if possible. With appropriate masking of thesubstrate surface, structures having a relatively large aspect ratio maybe created in this way.

The DRIE process is a time-controlled two-step process in which anetching step alternates with a passivation step. These two process stepsare regulated independently of one another. The etching step includes anisotropic chemical etching process, and an anisotropic physical materialremoval, which attacks essentially at a right angle to the substratesurface, superimposed on the chemical etching process. The DRIE processprovides for stopping the etching step after a short period of time tocreate a passivation layer on the masked and structured substratesurface. The vertical side walls of the etching recesses created inadvance are also passivated here in particular. Therefore the side wallsare protected against the chemical etching attack of the next etchingstep while the passivation layer on the horizontal surfaces of theetching recesses is removed again by the physical material removal, sothat a chemical etching attack may take place here. The sequence ofetching step and passivation step is repeated as often as needed untilthe intended structure depth is reached.

It is known that cavities may be created in the back side of a siliconsubstrate with the aid of the DRIE process, thereby exposing membranesin the substrate's front side. However, this procedure has proven to beproblematical in two regards. Since the DRIE process is atime-controlled process, the variance in the achieved etching depth andthus also the variance in the membrane thickness are relatively greatwith the given process parameters. Furthermore, the bottom of thecavity, i.e., the membrane's back side, is relatively uneven due to theprocess. Both of the aforementioned aspects have undefined effects onthe mechanical properties of the membrane created in this way. However,further processing of such a membrane has also proven to beproblematical. For example, the process management with subsequentstructuring depends to a significant extent on the membrane thickness.Furthermore, the use of lithographic methods for definition of themembrane structure requires a preferably planar membrane surface ormembrane's back side when the structuring is to take place starting fromthe substrate's back side.

In practice, membranes are often created in the front side of a siliconsubstrate in an anodic KOH back-side etching process. For this purpose,an n⁺-doped area extending over the entire membrane surface area andfunctioning as an etch stop limit for the KOH etching process is exposedin the top side of the substrate. Although membranes having awell-defined thickness and a very planar back side may be manufacturedin this way, the rear opening in the cavern thus created beneath themembrane must be much larger than the membrane surface area becauseopenings in the form of truncated pyramids are formed during KOHetching. This limits the miniaturization options of MEMS componentsmanufactured accordingly.

BRIEF SUMMARY OF THE INVENTION

A method for permitting the implementation of defined micromechanicalmembrane structures on an extremely small chip surface is proposedaccording to the present invention.

According to the present invention, a doped area is created for thispurpose in the substrate's front side, the depth of this areacorresponding to the intended membrane thickness and its lateral extentcovering at least the intended membrane surface area. Regardless ofthis, a cavity is created in the substrate's back side beneath the dopedarea in a DRIE process. This DRIE process is aborted before reaching thedoped area. The cavity is then deepened in a KOH etching process,whereupon the doped substrate area functions as an etch stop, so thatthe doped substrate area above the cavity remains as the basic membrane.

According to this, the present invention is based on a combination ofthe time-controlled DRIE process with an etch stop-limited KOH processin which the advantages of the one process compensate for thedisadvantages of the other. It has been recognized that with the aid ofthe DRIE process, caverns having a much higher aspect ratio may becreated in the back side of the silicon substrate than in a KOH etchingprocess. This advantage of the anisotropic DRIE process in comparisonwith KOH etching is utilized according to the present invention toachieve a very high degree of component miniaturization. According tothe present invention, however, the cavern should be finalized with theaid of a KOH etching process. In contrast with the time-controlled DRIEprocess, the KOH etching process may be limited by an etch stop createdin the substrate. This advantage of the KOH etching process incomparison with the DRIE process is utilized according to the presentinvention to manufacture membranes having a defined thickness, which isuniform over the entire membrane surface area. The lateral extent andthe thickness of the membrane are independent of the depth of thecavern.

The membranes exposed according to the present invention maysubsequently also be provided with structuring, depending on theapplication. It is advantageous in particular if such a structuringproceeds from the membrane's back side because the substrate's frontside may then be processed independently of the membrane. In this case,the membrane's back side, i.e., the bottom of the cavity, must be maskedaccordingly. It has proven to be particularly advantageous that the backside of a membrane exposed according to the present invention isextremely planar. This is a prerequisite for use of a lithographicmethod for definition of the membrane structure in the masking layer.The very planar membrane's back side allows the use of special lightingdevices having only a shallow depth of field but a very high resolution.

As already mentioned, the rear structuring of the membranes exposedaccording to the present invention offers the possibility of independentprocessing of the substrate's front side. In a particularly advantageouscontinuation of the method according to the present invention, at leastone sacrificial layer is applied to the doped substrate's front side tothen create a second membrane structure over this sacrificial layerusing the methods of surface micromechanics. This makes it possible toimplement MEMS components for a wide variety of applications having twoindependently structured membrane structures situated one above theother. These membrane structures may be linked mechanically and/orelectrically, depending on the function of the component.

The production of microphone structures may be mentioned here as anexample of the versatile possibilities for use of the method accordingto the present invention. With the aid of the lithographic methodapplied to the membrane's back side, vent openings in the basic membranemay be created in this case.

Another example is the production of acceleration sensor structures oractuator structures. In this case, spring structures, for example, maybe created in the basic membrane as a suspension for a seismic mass withthe aid of the lithographic method applied to the membrane's back side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a through 1 c each show a schematic sectional diagram of asilicon substrate in the course of the method according to the presentinvention for creating a membrane structure.

FIGS. 2 a through 2 c show a view of the top side, a sectional view anda rear view of an MEMS component before removal of the sacrificial layerbetween the basic membrane and the layer structure.

FIGS. 3 a and 3 b show a view of the top side and a sectional view ofthe MEMS component after removal of the sacrificial layer between thebasic membrane and the layer structure.

FIG. 4 shows a sectional view of another MEMS component in which thebasic membrane and another membrane are linked mechanically andelectrically via the basic membrane.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the present invention for creating amicromechanical membrane structure is directed to a silicon substrate.The exemplary embodiment described here is a p-doped silicon substrate 1in whose top side a membrane of a defined thickness and having a definedmembrane surface area is to be exposed. For this purpose, an n⁺-dopedarea 2, the depth of which corresponds to the intended membranethickness and the lateral extent of which is at least slightly largerhere than the intended membrane surface area, is created in the frontside of substrate 1. Next, in a DRIB (deep reactive ion etching)process, a cavity 3 is created in the substrate's back side beneathn⁺-doped area 2. The opening cross section of this cavity 3 must beselected to be somewhat larger than the intended membrane surface area.According to the present invention, the DRIE process is limited in timein such a way that cavity 3 does not extend all the way up to n⁺-dopedsubstrate area 2. FIG. 1 a shows substrate 1 after the DRIE process isaborted. Bottom 31 of cavity 3 is located here at a distance ofapproximately 10 μm-50 μm from the interface of n⁺-doped substrate area2. Due to the process, bottom surface 31 of cavity 3 is curved anduneven.

Cavity 3 is then deepened in an anodic KOH etching process. The n⁺-dopedsubstrate area 2 or its interface functions as an etch stop for thisetching process. Accordingly, n⁺-doped substrate area 2 remains abovecavity 3 as basic membrane 10, as illustrated in FIG. 1 b. The lowerarea of the side wall of cavity 3 has the inclination characteristic ofKOH etching, which is why the opening cross section of cavity 3 at therear has been selected to be somewhat larger than the intended membranesurface area. It is important here that the depth of cavity 3 and thusalso the thickness of basic membrane 10 may be predefined very preciselywith the aid of n⁺-doped substrate area 2. Bottom surface 32 of cavity 3is planar over the entire area, corresponding to the shape of the etchstop limit and is oriented in parallel with the substrate surface.

In the exemplary embodiment described here, basic membrane 10 is thenstructured starting from the substrate's back side—i.e., regardless ofany processing of the substrate's front side. The definition of themembrane structure is achieved with the aid of a masking layer, which isapplied to the back side of basic membrane 10 and is structuredaccordingly. For this purpose, the substrate's back side is providedwith a homogeneous photoresist layer in a spray lacquer process,covering bottom surface 32 of cavity 3 but not completely filling upcavity 3. The definition of the membrane structure is now accomplishedthrough corresponding lighting of the photoresist layer. The evennessand parallel alignment of cavity bottom 32 with respect to the substratesurface allow the use of special lighting devices having a very highresolution, although this is to the detriment of the depth of field.This also makes it possible to very accurately define and position evenextremely small openings, e.g., vent openings of microphone structures.FIG. 1 c shows basic membrane 10 after structuring of the back side inwhich through-openings 11 have been created.

On the basis of FIGS. 2 through 4, it is explained below as an examplethe possibilities offered by structuring the back side of the basicmembrane independently of processing the substrate's front side duringmanufacturing of MEMS components. Thus the method according to thepresent invention allows implementation of a second membrane structureabove the basic membrane. This second membrane structure is formed bymethods of surface micromechanics in a layer structure on the siliconsubstrate. For this purpose, at least one sacrificial layer is appliedto the substrate surface, preferably before any structuring of the basicmembrane, and is then removed again only after the structuring of thebasic membrane and after the creation of the second membrane structureabove the basic membrane, to expose the basic membrane and the secondmembrane structure.

As an example of such an MEMS component, FIGS. 2 and 3 show a sensorelement 20 with which accelerations in the z direction are detectable.FIGS. 2 a through c show the sensor structure before removal ofsacrificial layer 4, and FIGS. 3 a, b show the sensor structure afterremoval of the sacrificial layer.

Sensor element 20 was produced on the basis of a silicon substrate 1 andincludes a basic membrane 21, which is formed in the surface ofsubstrate 1 and spans a cavern 3 in the substrate's back side. Thiscavern 3 was created in a time-controlled DRIE process and a subsequentKOH etching process including an etch stop in the form of a dopedsubstrate area 2—as explained in detail in conjunction with FIGS. 1 a,b. Due to the DRIE process, the side wall in the opening area of cavern3 is oriented essentially at a right angle to the plane of thesubstrate. Only in the bottom region does cavern 3 assume the form of atruncated pyramid, which is attributed to the KOH etching process. Thischaracteristic of cavern 3 is illustrated in particular by the twosectional diagrams in FIGS. 2 b and 3 b. By structuring the membrane'sback side—as described in detail in conjunction with FIG. 1 c-a rockerstructure 22 has been formed in basic membrane 21. It includes twopaddles 221 and 222 of different sizes, which are connected to oneanother via a web-type spring structure 223 and are also connected tothe substrate frame. Spring structure 223 acts as a torsion spring. FIG.2 c shows the layout of basic membrane 21.

Each of the two paddles 221 and 222 functions as a movable electrode ofa measuring capacitance with which the deflection of paddle 221 or 222is detected. Stationary counterelectrodes 251 and 252 of the measuringcapacitances are implemented in an epipolysilicon layer 5 via rockerstructure 22. For this purpose, an oxide layer was initially created assacrificial layer 4 on the substrate surface and then structured. Next,a thick epipolysilicon layer 5 was deposited on structured sacrificiallayer 4. An electrical contact 23 between epipolysilicon layer 5 andsubstrate 1 was formed at the side of basic membrane 21. Finally,epipolysilicon layer 5 was structured in a front-side etching process inwhich insulation trenches 26 for the definition of counterelectrodes 251and 252 were produced on the one hand, and on the other hand ventopenings 27 were produced to improve the damping properties of sensorelement 20. The layout of epipolysilicon layer 5 is shown in FIG. 2 a.This diagram also shows the configuration of bond pads 28 for electricalcontacting of sensor element 20.

Only after basic membrane 21 and epipolysilicon layer 5 have beenstructured independently of one another is sacrificial layer 4 removedin the area above basic membrane 21 to expose rocker structure 22. Asecond membrane structure 25 having two stationary counterelectrodes 251and 252 is then formed in epipolysilicon layer 5. FIGS. 3 a and 3 b showthat both counterelectrodes 251 and 252 are rigidly connected tosubstrate 1 in the area of bond pads 28 and on the opposite side viasacrificial layer material 4. The etching attack to remove thesacrificial layer material may start from the substrate's back side viacavern 3 and the openings in basic membrane 21 or may also take place ina front-side etching process via insulation trenches 26 and ventopenings 27 in epipolysilicon layer 5.

The configuration of an MEMS component having a basic membrane in thesilicon substrate and a second membrane structure above the basicmembrane, which is formed in an epipolysilicon layer, allows bothmechanical and electrical coupling between the basic membrane and thesecond membrane structure. This will now be explained with reference toFIG. 4, which shows a component 40 having a basic membrane 41 and asecond membrane structure 45 above basic membrane 41. As in the case ofsensor element 20, basic membrane 41 was created and structured byprocessing the substrate's back side, and second membrane structure 45was structured out of a thick epipolysilicon layer 5. However, in thecase of component 40, sacrificial layer 4 was structured not only overthe boundary area of basic membrane 41 but was also structured directlyover basic membrane 41, so that epipolysilicon layer 5, which was grownthereafter, was in contact with silicon substrate 1 not only via a firstelectrical contact 42 next to the basic membrane but also secondmembrane structure 45 is connected to basic membrane 41, namely via asecond electrical contact 43. Furthermore, dielectric sacrificial layer4 was not removed entirely, so that basic membrane 41 and secondmembrane 45 are additionally linked mechanically via remainingsacrificial layer columns 44.

1. A method for manufacturing a micromechanical membrane structure,comprising: providing a cavity in a back side of a silicon substrateusing a DRIE (deep reactive ion etching) process; providing a doped areain a front side of the substrate, wherein the depth of the doped areacorresponds to the thickness of a desired membrane and the lateralextent of the doped area covers at least the surface area of the desiredmembrane; aborting the DRIE process before the bottom of the cavityreaches the doped area; and deepening the cavity using a KOH etchingprocess in which the doped substrate area functions as an etch stop, sothat the doped substrate area remains over the cavity as a basicmembrane.
 2. The method as recited in claim 1, wherein the DRIE processis aborted when the distance between the bottom of the cavity and thedoped area is approximately 10 μm-50 μm.
 3. The method as recited inclaim 2, wherein the basic membrane is structured with the aid of alithographic process applied to the membrane's back side.
 4. The methodas recited in claim 2, wherein at least one sacrificial layer is appliedto the doped front side of the substrate, and wherein at least oneadditional membrane structure is created over the sacrificial layerusing surface micromechanics process.
 5. An MEMS component, comprising:a first membrane structure positioned over a cavity formed in a siliconsubstrate's back side, wherein the side walls of the cavity are orientedessentially perpendicularly to the substrate's back side, and wherein abasic layer of the first membrane structure is formed within a dopedarea in the substrate, and wherein the sides of the cavity taper in theform of a truncated inverse pyramid toward the bottom of the cavity, andthe bottom surface of the cavity is planar and is oriented essentiallyin parallel with the substrate surface.
 6. The MEMS component as recitedin claim 5, further comprising: a second membrane structure formed in alayer structure over the first membrane structure, the first and secondmembrane structures being structured independently of one another. 7.The MEMS component as recited in claim 6, wherein the first membranestructure and the second membrane structure are at least one ofmechanically and electrically linked.
 8. The MEMS component as recitedin claim 6, wherein the MEMS component has a microphone structure, andwherein the first membrane structure includes vent openings.
 9. The MEMScomponent as recited in claim 6, wherein the MEMS component has one ofan acceleration sensor structure or an actuator structure, and whereinthe first membrane structure includes at least one spring structure as asuspension for at least one seismic mass.